The present disclosure is directed to an improved detector and especially one which can be used in measuring while drilling (MWD) apparatus. It also can be used in wireline supported logging devices. As a generalization, the present disclosure is directed to an improved detector which has improved capability for distinguishing randomly occurring nuclear events from events which are primarily resultant from vibrations of the equipment. In particular, with the advent of thermal neutron porosity tools which are typically now mounted in MWD equipment, thermal neutron detectors are placed in the drill collars. One type of thermal neutron detector is a chamber which is filled with a gas sensitive to the transient neutrons passing through the container. For instance, one typical detector system utilizes a chamber filled with .sup.3 He. This gas filled detector has a known thermal neutron absorption cross section. This type detector will respond to the randomly occurring neutron events, but it will also regrettably respond to environmentally created vibrations or shocks inherent in use of the equipment.
Particularly in a MWD system, the equipment is normally mounted in a thick walled pipe which is otherwise known as a drill collar. The drill collar is normally included in the drill stem just above the drill bit and below the drill pipe to add weight for the drill bit and to also provide a stiffer pipe to assure that the well remains true without drift. The stiffness of the pipe in this circumstance is engendered in part by the thick wall construction. The MWD equipment including the appropriate circuitry and the detectors are placed in recesses in the thick wall. When so mounted, the MWD equipment is immediately adjacent to the drill bit so that it can make measurements of the formations as they are penetrated by the drill bit. However, positioning of this structure and mounting of the MWD circuitry and detector in a drill collar which is threaded to the drill bit subjects the detector to substantial shock impact. As a generalization, the shock occurring at the lower end of a drill stem is quite severe. While the drill bit remains on the bottom during rotation, it nevertheless is impacted with a constant vibration dependent on the number of teeth in the cones making the drill bit and the speed of rotation. This establishes a fundamental frequency vibration with harmonics or overtones which are felt or sensed in the drill stem and particularly at the MWD equipment. Separate from all of this there are, however, random occurring events which provide vibrational shock to the drill bit which shock is coupled into the MWD equipment. While some of this shock may be repetitive, it is also mixed with randomly occurring vibrational shock to the MWD equipment. While shock is worse with MWD equipment because it is near the drill bit which bears against the lower end of the well, there is a similar risk of vibration in a wireline tool. Typically, a wireline tool is lowered as fast as possible into a well borehole until it arrives at the bottom. It is then retrieved at a fixed speed along the well. Data is normally collected during retrieval of the wireline supported tool. In this instance, there again is the risk of vibration by bumping and banging of the tool as it is raised in the well borehole. This banging is manifest by forming vibrational forces in the wireline supported tool which impinge on the thermal neutron detector.
Detectors are normally intended to form an output signal when a thermal neutron traverses the detector. However, the vibration forces described above are quite substantial and may indeed create spurious output signals. Indeed, the instantaneous shock or impact loading which occurs in a wireline supported tool can generate shock waves of a few g's. In a MWD tool, the vibrational forces can be larger, perhaps ten, twenty or even thirty g's. When this occurs, the detectors normally form an unwanted output which is spurious. Not only is it spurious, it is of such large amplitude that it may well obscure thermal neutron events which should have been captured and recorded. This kind of shock originated output noise signal is described as microphonics. The present disclosure sets forth a method and apparatus for reducing or eliminating the microphonics so that the neutron originated events are observed and can be recorded.
Current techniques for reducing microphonics focus primarily on improved mounting for the detectors such as shock absorbers and the like. Alternately, electronic filtering of the microphonic signals is effective. It is submitted that the method and apparatus of this disclosure reduces microphonics markedly.
The present disclosure particularly sets forth a system for MWD construction where two detector are incorporated. The two detectors are mounted in close proximity (e.g., side by side) so that they are, exposed to the same vibrational forces. The output signals are provided to a differential amplifier which subtracts one from the other. The vibration induced noise signal occurs commonly in time in both detectors whereas the nuclear events are random. Using principally the common mode rejection obtained from differentially coupling these signals, the vibrationally induced signal is cancelled. Depending on the frequency of the nuclear events and other system factors, the microphonic signals from the two detectors are differentially cancelled with only a small portion of the data lost. In other words, the differential output signal has the neutron components minimally reduced while the component attributable to the microphonics is eliminated, ideally only leaving the neutron signals.
In this configuration, differential cancellation of the microphonic noise is substantially accomplished. This assumes that the two detectors are equal in all regards. It is possible, however, to fill one of the two detectors with the .sup.3 He gas in the conventional fashion, and to fill the other detector with a gas which has a markedly reduced neutron absorption cross section, wherein that detector has its neutron sensitivity substantially reduced to zero, or nearly so. This detector would in that instance only generate a microphonic response signal which, on being scaled in the proper fashion, could then be used as a differential offset to the signal from the other of the two detectors. The foregoing is directed to a two detector system. An alternate embodiment of the present disclosure utilizes similar and adjacent anode wires in a single detector chamber. The two conductors or wires in the central portion of the system move together, meaning they respond in similar fashion to the shock loading and therefore the two output signals are provided to a differential amplifier. When a nuclear event then occurs, the output typically is found at only one of the two anode wires and the intended output is substantially not impacted by the incorporation of the differential amplifier in the two anode wire system just mentioned. In another embodiment, the two wires are mechanically joined to assure common motion. For instance, the wires are joined by an insulator such as an epoxy resin bridge.